It is the index that the soil survey map of the INTA bring in the property «ind_prod». It condenses into a single number what the class of land capability says in categories: how good the land is for producing.
It is built by combining the properties that limit performance—depth, texture, drainage, salinity, slope, rainfall regime—and penalizing each limitation. The result is expressed over 100, where 100 would be soil without any restriction for the crop.
Zero is a category, not the worst soil
An index of 0 does not mean "the worst land in the country": it means that the soil survey map marked that unit as unsuitable for agriculture. There are 631 of the 5,322 units of the national soil survey map, and soil survey map itself says it explicitly in its property main limitation. That is why zero has its own section on the scale and is not mixed with low indices: they are different things.
How to calculate the value of the integer property
A property almost never falls within a single unit of soil. The index shown by report is the average of the units that cover it, weighted by the surface area each one occupies: a unit that covers 70% of property weighs seven times more than one that covers 10%.
Next to the index is always the coverage percentage. If the soil survey map only covers 60% of the property, the index was calculated on that 60% and must be read knowingly.